1. Field of the Invention
The present invention relates to neuron devices.
2. Related Art
To improve the performance of a semiconductor integrated circuit, the field effect transistors that form the semiconductor integrated circuit must exhibit high performance. The essential requirement for high-performance devices is miniaturization of devices, and so far, the performance of devices has been improved by miniaturizing the devices. However, as the devices became smaller, the technique problems to be solved became larger. According to the international semiconductor roadmap, very large technical difficulty is expected to accompany the development of the 45 nm generation and later generations.
In these circumstances, various techniques for improving performances other than miniaturization have been studied. One of the suggestions is the technique for achieving a high-performance system by employing MOS field effect transistors called neuron MOS transistors (hereinafter also referred to as neuron devices) as the basic gates instead of conventional transistors (see JP-A 2001-266106 (KOKAI), for example). Each of such MOS field effect transistors has more than one gate and a floating gate.
However, each neuron MOS transistor employed by the conventional techniques has poorer switching characteristics than conventional MOS transistors. Therefore, if a system that can be formed with conventional MOS transistors is formed with neuron MOS transistors, such a system has few advantages. As a result, the range of application of neuron MOS transistors is limited to the application developed especially for neuron MOS transistors. More specifically, having a small short-channel effect immunity, conventional neuron MOS transistors cannot be miniaturized. Also, since the conventional neuron MOS transistors exhibit low channel charge density during ON operations, the current drivability cannot be made larger.
As described above, the conventional neuron MOS transistors have the difficulty in miniaturization and small current drivability. This is because the potential of the channel is controlled by the capacitance coupling in each conventional neuron MOS transistor. Therefore, it is difficult to reduce the thickness of each gate insulating film in an equivalent manner, and as a result, the short-channel effect immunity and the channel charge density become lower.